The first microgravity combustion experiments were carried out in 2017 aboard the Japanese Experiment Module “Kibo“ on ISS, titled “Elucidation of Flame Spread and Group Combustion Excitation Mechanism of Randomly Distributed Droplet Clouds (Group Combustion).“ The flame spread over a large-scale droplet cloud consisting of about 100 droplets randomly distributed on a SiC-fiber lattice was observed. When droplets are generated one by one, the diameter of a droplet generated earlier changes more during large-scale droplet-cloud generation. The droplet pre-vaporization effect needed to be well understood to generate large-scale droplet clouds with a uniform initial droplet diameter for “Group Combustion“ experiments. We describe the concept of the large-scale droplet-cloud generation and report the vaporization-rate constants for a single droplet, for two interactive droplets and for a droplet cloud with n-decane as a fuel and verification results obtained in the “Group Combustion“ experiments aboard ISS. The evaluation of the vaporization-rate constant during the droplet-cloud generation suggests that the droplet vaporization in the droplet cloud occurred with interaction among many more than three droplets. Considering the pre-vaporization with the vaporization-rate constant estimated, most droplets had a nearly uniform diameter at ignition. Satisfactory reproducibility was confirmed in the droplet diameter at ignition and the flame-spread behavior.
As a preliminary experiment of the “Group Combustion” experiment conducted in the Japanese Experiment Module “Kibo“ aboard the International Space Station (ISS) in 2017, a combustion experiment of an unevenly arranged droplet cloud with 148 n-decane droplets tethered on a SiC-fiber lattice was conducted in microgravity during parabolic flight of an aircraft. In this study, three methods of identifying the local flame-spread direction were proposed to determine the local flame-spread rate between two adjacent droplets of an unevenly arranged droplet cloud considering the local flame-spread direction. The first method employed the vertical direction from the isothermal line to the unburned droplet. The isothermal line was approximated based on the luminance of SiC fibers tethering droplets. The second method considered the luminance distribution of the initial flame immediately after ignition. The third method compared the local flame-spread-rate candidates from neighboring droplets with the flame spread-rate of the linear droplet array. It was possible to identify the local flame-spread direction by any one of these methods, and the local flame-spread rate is calculated between two droplets close to this direction. The results suggest that there are many conditions under which the local flame-spread rate of an unevenly arranged droplet cloud is greater than the flame spread rate of the linear droplet array.
Viscosity measurements for SiO2-CaO-Al2O3 based ternary slags with low SiO2 content were performed for a wide temperature range utilizing the aerodynamic levitation and rotating bob methods. Aerodynamic levitation was used for temperatures ≥2229 K and the viscosity was calculated by the sample oscillation decay time. The rotating bob method was used for temperatures ≤1898 K and the viscosity was determined by the variation of the torque at different rotation speeds. Fitting curves were created using Mauro’s viscosity equation. The main sources of systematic error were identified to be the sample weight measurement, the resolution of the high-speed camera, the fitting of the linear trend line in the torque against rpm diagrams and the vertical position of the bob. The combined standard uncertainty from all error sources was calculated for both measurement methods.
An electrostatic levitation furnace for microgravity experiments has been developed and transferred to the International Space Station (ISS). This electrostatic levitation method utilizes Coulomb force between a charged sample and surrounding electrodes for sample position control. On the ground experiment, amount of charge accumulated on the sample surface has to be 10-9 to 10-10 C to levitate it against gravity. In microgravity, it was found that a sample can be stably handled even though the amount of surface charge is 10-13 C. When the control parameters are properly chosen, the position stability of the sample during heating can be maintained better than ±100 µm. This stability is good enough to precisely measure the sample temperature by a pyrometer as well as to measure the sample volume by an image analysis.